A fossil fuel such as coal includes heavy metal which is harmful to a human body although an amount thereof is very small. When the fossil fuel is burned, most of them become gas. In a thermal power plant or the like, after most of nitrogen oxide (NOx), sulfur oxide (SOx) and smoke and dust in the exhaust smoke generated when coal or the like is burned is removed, the remainder thereof is discharged to atmosphere from a smokestack. However, a portion of the heavy metal is not completely removed by a dust collecting apparatus which collects smoke and dust in the exhaust smoke or by a desulfurization apparatus which removes sulfur oxide. Examples of elements of heavy metal included in coal which are easily discharged from smokestack are mercury, selenium, arsenic, chromium, lead and the like having high volatility. Although the concentration of these elements in the exhaust smoke when they are discharged from the smokestack is not high, it is desired that its processing technique becomes widespread, since their toxicity is high. For example, there is proposed a mercury processing method in the exhaust gas discharged from a refuse incinerator as a method for removing mercury in the exhaust gas (Japanese Patent Publications Nos. H6-61424 and H6-104182). This is because that higher concentration mercury is included in exhaust gas from the refuse incinerator.
According to the techniques disclosed in the above publications, liquid absorbent or solid absorbent is sprayed into exhaust gas, mercury in the exhaust gas is collected by the absorbent, and the absorbent which collected the mercury together with dust in the exhaust gas is collected by a dust collecting apparatus such as a downstream bag filter or a electrostatic precipitator. It is proposed that powder activated carbon is sprayed upstream of a bag filter which removes mercury, and it is collected by a downstream bag filter (Felsvang K. et al.: Activated carbon injection in spray dryer/ESP/FF for mercury and toxics control: Fuel Process Tech. 39 PP. 417-430 (1994)).
According to these conventional techniques, however, an apparatus which sprays liquid absorbent or solid absorbent into exhaust gas, and a reactor which brings liquid absorbent or solid absorbent and exhaust gas into contact for a certain time (normally several seconds) are required, and it is necessary to secure the installation space therefore. Further, liquid absorbent or solid absorbent is expensive., and there is a problem that the processing cost is increased.
That is, according to the conventional techniques, consumption amount of expensive liquid absorbent or solid absorbent is large, and the reactor and the apparatus for spraying liquid absorbent or solid absorbent into exhaust gas are required, and it is necessary to secure the installation space therefore. It is an object of the present invention to propose an exhaust smoke processing system capable of solving these problems and capable of economically removing heavy metal.